You can cite in any way you want, but we provide a special way to site books by ISBN. Siafoo uses Amazon's API to look up the title of the book and to provide a link to Amazon. Why use the ISBN and not just the title? Two reasons:

Anyone reading will know exactly which version of the book you are talking about. The worst is to buy a book and realize that what you wanted was only in the next/previous edition.

We get a small commission for each book bought through one of these links, so you are supporting Siafoo. :)